X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-07-09
2002-08-13
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S020000
Reexamination Certificate
active
06434214
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an X-ray computed tomography, and more particularly to the technique which is effectively applied to the positioning of a rotation center of a scanner of a detection system in a cone-beam X-ray computed tomography.
BACKGROUND ART
FIG. 6
is a view showing the general construction of a conventional cone-beam X-ray CT. The conventional cone-beam X-ray CT was divided into imaging unit
1
for carrying out the imaging and image processing unit
2
for processing the detected image data. Control unit
3
carries out the whole control for the imaging unit
1
and the image processing unit
2
. In the imaging unit
1
, an X-ray source
5
and a two-dimensional detector
6
were arranged in such a way as to be opposite to each other through an object. The X-ray source
5
and the two-dimensional detector
6
were both arranged in a scanner as a scanning mechanism which is rotated around an object
7
with a central axis
9
for the rotation as the rotation center.
The scanner
4
was rotated every predetermined angle and the two-dimensional detector
6
carries out the measurement of the intensity of the transmitted X-rays, which were transmitted through the object
7
, every predetermined angle, thereby carrying out the imaging of the transmitted X-ray image of the object
7
. The transmitted X-ray image which has been imaged by the two-dimensional detector was converted into the digital image data which was in turn outputted to the image processing unit
2
. But, in the following description, the angle of rotation of the scanner
4
is referred to as a projection angle a.
In the image processing unit
2
, first of all, the pre-processing such as the gamma correction, the distortion correction, the logarithmic transformation and the non-uniformity correction of the two-dimensional detector
6
were carried out in pre-processing means
10
. Next, reconstruction means
11
, on the basis of all of the transmitted X-ray images (all of the projected images) after completion of the pre-processing reconstructed the three-dimensional reconstructed image which is the three-dimensional X-ray absorption coefficient distribution within the field of view of the object
7
. As for this reconstruction arithmetic operation method, there is known the cone-beam reconstruction arithmetic operation method or the like by Feldkamp described in an article of L. A. Feldkamp et al.: PRACTICAL CONE-BEAM ALGORITHM, Journal of Optical Society of America, A.Vol. 1, No. 6, pp. 612 to 619 (1984) (article 1).
Finally, imaging means
12
subjected the three-dimensional reconstructed image to the image processing such as the volume-rendering processing or the maximum-intensity-projection processing of displaying the resultant image in the form of the two-dimensional image on display means
13
. At this time, on the basis of the parameters of a viewpoint, a region and the like to be observed which has been inputted through instruction means (not shown) such as a keyboard, a mouse and a track ball, the imaging means
12
executed the image processing.
In the conventional cone-beam X-ray CT, the scanner
4
mounted with the imaging system including the X-ray source
5
and the two-dimensional detector
6
was rotated and the transmitted X-ray image obtained around the object
7
was imaged, and the reconstruction means
11
obtained the three-dimensional X-ray absorption coefficient distribution of the object
7
placed on the stationary coordinate system fixed to the apparatus body. The stationary coordinate system was defined by the imaging system, i.e., the Z-axis as the rotation center
9
of the scanner
4
, and the rectangular Cartesian coordinates on the plane on which the rotation orbit of an X-ray focus
14
of the X-ray source
5
lies (hereinafter, referred to as “a mid-plane” for short, when applicable), i.e., the X-axis and the Y-axis.
The position of an X-ray beam
8
imaged by the detection elements of the two-dimensional detector
6
was specified by an angle (projection angle) a between the straight line which passes through the orbit of the XYZ coordinate system for the X-ray focus
14
to reach the two-dimensional detector
6
and the X-axis, “the rotation-axis projection” which was obtained by projecting the rotation center
9
on an imaginary plane (projection plane)
15
is placed on the incident plane of the two-dimensional detector
6
, and “the mid-plane projection” which is the straight line drew by the intersection between the mid-plane and the projection plane. That is, the coordinate axes, when reconstruction the three-dimensional X-ray absorption coefficient distribution of the object
7
, were the rotation axis projection and the mid-plane projection on the projection plane.
Since for the actual imaging of the transmitted X-ray image, the continuous analog imaging is not carried out, but the discrete digital imaging is carried out, when performing the reconstruction arithmetic operation, the sampling pitch DP on the projection plane was also required. In addition, a distance SOD extending from the X-ray focus
14
to the rotation center
9
, and a distance SID extending from the X-ray focus
14
to the rotation-axis projection
17
were both required. In the following description, the relative positional relationship among the X-ray focus
14
, the two-dimensional detector
6
and the rotation center
9
will be referred to as “the geometry of the imaging system”. The geometry of the imaging system is defined by the distance SOD extending from the X-ray focus
14
to the rotation center
9
, and the distance SID extending from the X-ray focus
14
to the rotation-axis projection
17
, the sampling pitch DP, the rotation-axis projection and the mid-plane projection on the projection plane
15
.
It is well known that of the parameters by which the geometry of the imaging system is determined, the higher accuracy is required for the rotation-axis projection, the mid-plane projection and the sampling pitch than for the the distance SOD extending from the X-ray focus
14
to the rotation center
9
, and the distance SID extending from the X-ray focus
14
to the 17 rotation-axis projection. For example, when the effective aperture width of the two-dimensional detector
6
is 30 cm, and the resolution thereof is 512×512 pixels, the accuracy of the rotation-axis projection, the mid-plane projection and the sampling pitch DP required 0.1 pixel, i.e., about 0.05 mm. This reason is that even if the fine error is present in the positions of the rotation-axis projection and the mid-plane projection, and the sampling pitch DP, the reduction of the image quality is provided for the reconstructed image.
It is known that of the positions of the rotation-axis projection and the mid-plane projection, and the sampling pitch DP, in particular, the rotation-axis projection is important, and even if the fine error is present, generates the remarkable artifact in the reconstructed image. On the other hand, it was difficult to image directly the positions of the rotation-axis projection and the mid-plane projection, and the sampling pitch DP. This reason resulted from the fact that the positions of the rotation-axis projection and the mid-plane projection, and the value of the sampling pitch DP depend on the characteristics of the two-dimensional detector
6
and the installation state of the apparatus.
As for a method of imaging the geometry of the imaging system with high accuracy, there was “the X-ray Tomographic Imaging System” described in JP-A-9-173330 (article 2) by the same applicant. In the X-ray tomographic imaging system described in the article 2, first of all, an object (phantom)
19
including a support member
20
and a corpuscle-shaped high absorption member
21
shown in
FIG. 7
is arranged in the vicinity of a rotation center
9
(in the position which 3 cm to several centimeters a way from the rotation center
9
), and the transmitted X-ray image thereof is imaged from the all-round direction. But, in the following description, the dedica
Kawai Hiroyuki
Okajima Ken'ichi
Gemmell Elizabeth
Hitachi Medical Corporation
Porta David P.
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